This invention relates to circuits for providing rapid turn-off of transistors and gate turn-off thyristors (gto's).
FIGS. 1a and 1b of the accompanying drawings show typical transistor switching circuit configurations using an npn transistor T1 (FIG. 1a) and a pnp transistor T2 (FIG. 1b) respectively. The transistor T1 or T2 is turned on by the application of current across the transistor base-emitter (b-e) junction, in the direction shown by the "on" arrow in FIGS. 1a and 1b. This causes the voltage across the transistor T1 or T2 to fall to near zero and the supply voltage V to be applied across the load causing current to flow from the supply V through both the load and the transistor T1 or T2. In each circuit example the transistor emitter terminal (e) remains at a fixed reference potential, either OV in the case of FIG. 1a or +V in the case of FIG. 1b. Removal of the forward base current will cause the transistor T1 or T2 to turn off. The diode D provides a path for the load current when the transistor T1 or T2 is turned off. For fast turn-off, reverse base current is applied, as shown by the "off" arrow in FIGS. 1a and 1b, which causes the base-emitter junction to cut off very quickly.
Circuits of this type providing forward (on) and reverse (off) base current drive are known. The design of such circuitry is made simple by the fact that the transistor emitter terminal is always at a fixed and stable potential OV or +V.
FIGS. 2a and 2b of the accompanying drawings show two further known transistor switching circuits using an npn transistor T1 (FIG. 2a) and a pnp transistor T2 (FIG. 2b) respectively, but in these cases the transistor emitter terminal is not at a fixed potential OV or +V. In these cases both the base and emitter change potential (relative to, say, the OV rail) when the transistor T1 or T2 switches on and off. The base and emitter are said to be at floating potentials. This effect necessitates more complex circuitry to control the transistor T1 or T2. It is therefore unfortunate that FIG. 2a forms the basis of a very commonly required configuration in bridge circuits. Complex and costly techniques exist for providing the necessary floating on and off base drive for the configurations shown in FIGS. 2a and 2b. These techniques usually involve the use of floating power supplies, optically transmitted control signals, pulse transformers, etc, all of which are necessary to provide voltage isolation for the base and emitter with respect to the OV or +V voltage rails.
A simple turn-on technique exists for the configurations of FIGS. 2a and 2b, as shown in FIGS. 3a and 3b. This involves the use of a pnp-npn Darlington transistor configuration comprising a pnp transistor T3, an npn transistor T5 and resistors R1 and R2 (FIG. 3a), or an npn transistor T4, a pnp transistor T6 and resistors R3 and R4 (FIG. 3b). The switching transistors T3, T4, T5 and T6 can be replaced by field-effect transistors, and the transistor T1 in FIG. 3a can be replaced by a gto, in possible variation of this circuitry. Turn-on is achieved by switching transistor T5 or T6 on by application of a turn-on current across the base-emitter junction which turns on transistor T3 or T4 which, in turn, turns on the main transistor T1 or T2. Simple turn-on is thereby achieved but no reverse base current can be provided for fast, efficient turn-off. This is a particular disadvantage where the device to be controlled is a gto since gto's must have reverse gate current for turn-off.
It is an object of the invention to provide a simple technique for achieving rapid turn-off of a bipolar or field-effect transistor or gate turn-off thyristor (gto) having an emitter or cathode at a floating potential utilising a reverse base or gate current.